The present invention relates to an improved method for preparing dielectric ceramic compositions, and, more particularly, to a method for preparing a ceramic composition containing hollow microspheres which is castable on a substrate in the form of a tape for multilayer circuits.
Multilayer electronic circuits are widely used to increase circuit functionality per unit of area. Such circuits are normally made by first casting substrates in the form of sheets or a tape of unfired (green) ceramic, which are punched with via holes, screen printed with circuit metallurgy, laminated into a monolithic three-dimensional structure, and sintered. The ceramic substrate and metallurgical circuit components both densify simultaneously in the same firing cycle.
In order to take full advantage of the very high switching speeds of present day circuitry, faster signal transport through interconnects is necessary. The signal time delay of an electromagnetic wave on the substrate surface is a function of the dielectric constant of the substrate, and, consequently, a low dielectric constant is highly desirable.
Virteous silica has the lowest dielectric constant, 3.8-4 at 1 MHz of known silica phase ceramics, but high temperatures required for densification will normally preclude use of conventional conductive materials such as gold, silver and copper.
Lower sintering temperatures can be used to make dense substrates from many glass powders, but dielectric constants are in the 4-8 range at 1 MHz. To take full advantage of maximum available switching speeds, a substrate dielectric constant of 3 or less at 1 MHz is desirable.
One way to achieve a low substrate dielectric constant is to disperse hollow ceramic microspheres throughout the substrate. Composites made from glass powder and hollow alumina microspheres have dieletric constants generally in the range of 4-5 at 1 MHz, and those made using hollow silica microspheres can be in the range of 3.3-4 at 1 MHz.
Published Japanese patent application No. SHO 57[1982]-220331, "Circuit Substrate With Low Dielectric Constant", for example, describes a circuit substrate obtained by sintering a ceramic powder feed matrix having inorganic hollow spherical particles dispersed therein. The hollow particles comprise alumina (Al.sub.2 O.sub.3), but no processing techniques for preparing the matrix are described.
U.S. Pat. 4,141,055 describes a crossover structure for microelectric circuits which utilizes a dielectric material which includes spheres of an insulating material in order to separate the conductors. A UV curable epoxy including hollow glass spheres is deposited on the crosspoint areas followed by deposition of the crossover connector. In the preparation of the dielectric, the epoxy and the hollow spheres are mixed first by hand and then in a standard 3 roll mixer along with a silica filler.
Marcia J. Leap et al., Materials Research Laboratory, The Pennsylvania State University, published a paper entitled "Low Permittivity Silica Hollow Glass Microspheres 0-3 Composites" which describes the processing and dielectric properties of glass/air composites formed by sintering silica hollow glass microspheres. Specimens were prepared from a batch consisting of 10 vol. % lead glass powder and 90 vol. % SiO.sub.2 hollow glass microspheres. The powders were dry mixed manually, and, after addition of a binder solution, the mixture was dried and then milled for 5 minutes. SEM micrographs revealed that a large percentage of spheres had broken during milling.
The methods of the prior art have not taken full advantage of using microspheres for improving dielectric ceramic compositions. The procedures which have been used can result in crushing and breaking the shells of the microspheres and consequent loss of hermeticity/increase in dielectric constant. Thus, for a given composition, the optimum dielectric constant may not be obtained, and, due to a loss of hermeticity, there may also be an increased chance of failure in the multilayer electronic circuits in which these ceramics play a critical role.
In order to consistently make high quality low dielectric ceramic substrates, suitable for use with conventional conductive materials, a procedure is needed which will yield a uniform, hermetic distribution of intact hollow microspheres throughout a dense glass matrix.